1. Field of the Invention
The present Invention relates to a plasma generator for generating a cold plasma jet beam in the application field of medicine and in particular of surgical applications as well as in other fields of science.
2. Brief description of the Background of the Invention Including Prior Art
Plasma sources, which are capable of generating a plasma under atmospheric pressure conditions and having a temperature of above 1000 degrees centigrade belong to the general state of the art. The heating of a plasma is effected by classic electromagnetic induction, and in fact by the inductive heating of electrically conducting media in an alternating electromagnetic field of an induction coil. This plasma source comprises a high voltage alternating current generator, a tubular shaped quartz container and a liquid cooled induction coil, wherein the induction coil exhibits a large number of windings. A coil of 10 cm diameter is absolutely necessary to be furnished with many windings in order to be able to generate an induction in a volume of about one liter. The inductive resistance increases with frequencies of more than 1 MHz very much. It is difficult in general to adapt the high voltage generator to the inductive resistance because the effective output power is thereby decreased. The development of a plasma source according to the state-of-the-art effect leads to a reduction of the dimensions, however, also to a lesser effectiveness and to high plasma temperatures, which means a limitation of the field of application in particular in the field of medicine, biology and ecological treatments.
Other plasma generators are employed in electrical discharge apparatus, wherein the electrical discharge apparatus is equipped with an electronic oscillator, a modulation unit, a resonance transformer, and a rod shaped discharge electrode. A plasma jet beam is generated here between the end of the electrode and the object. The electronic oscillator generates oscillations of the order of magnitude of from about two hundred to 300 kHz. The modulator switches the oscillator signal on and off in a frequency region of from about three kHz to 5 kHz. Thus the modulator generates a densified excitation of the resonance transformer and of the object. This apparatus leads to a favorable therapeutic effect for the treatment of numerous illnesses mainly by applying heat to certain regions of the body.
This apparatus has however uncovered insufficiencies of the kind that the discharge transformation from the corona to the shape of the spark becomes visible and wherein the conduit leads directly into the tissue.
The classical corona and the spark discharge result unavoidably in ionization, electrical breakdown and heating of the air gap between the electrode and the body employed as a second electrode. A heat conditioned chaotic motion of particles in the discharge region is associated herewith.
The conventional radio frequency surgery works with a radio frequency generator in case of the spray coagulation or, respectively, argon coagulation (plasma discharge in an argon jet beam), wherein the radio frequency generator operates according to the method of shock impulse excitation and impact ionization of oscillating circuits. This impulse excitation of an oscillating circuit delivers short radio frequency pulses (bursts) with a pause interval of a relatively long time duration with respect to the pulse duration. The ratio pulse time duration/pause interval duration amounts to generally about 10 to 20 percent. This leads to a high peak power during the bursts and furthermore very high peak currents, which can reach several amperes. The capacity between patient and ground is insufficient under these conditions to close the current circuit. For this reason a neutral electrode is necessarily required in conventional radio frequency surgery. Furthermore the high currents result in a relatively large contact area of the electric arc on the tissue of several square millimeters and consequently are associated with a relatively large penetration depth of the heat effect. In addition, the flowing through of high peak currents through the tissue leads to a high current load even in larger depths, which can lead to damages at sensitive tissues, for example nerve tissue in the brain. The spectrum of a radio frequency current, which consists out of short bursts, comprises always a not to be underestimated part, which part is derived from the pulse repetition frequency. This frequency is generally in the range of between 10 and 70 kHz. These low frequencies can cause damage at sensitive tissues. Plasma generators of conventional kind, which generate the plasma inductively or by an arc discharge are too heavy because of the necessary cooling requirements for this purpose to be placed into a hand piece. Furthermore the conventional plasma generators exhibit a very high plasma temperature of up to several thousand degrees centigrade as a thermal plasma.
A further plasma generator is described in the European printed patent document EP-A 0837622. This generator comprises a voltage source, an electrical oscillator, wherein the electrical oscillator is furnished with an amplifier connected to a low voltage device, a resonance transformer furnished with a low voltage input and a high voltage output, and a connection of the high voltage output of the resonance transformer to the discharge electrode. This oscillator is furnished with a feedback winding, wherein the feedback winding is furnished for the operation with a single stage power oscillator. The very low stability of the discharge amplitude and the therewith associated problems of the power control are disadvantageous in connection with such a single stage oscillator.
U.S. Pat. No. 4,781,175 to McGreevy et al. teaches an electrosurgical conductive gas stream technique of achieving improved eschar for coagulation. A predetermined ionizable gas in a jet is conducted to the tissue at a predetermined flow rate sufficient to clear natural fluids from the tissue and to expose substantially the tissue stroma. To achieve fulguration electrical energy is conducted as arcs in the ionized conductive pathways.
The European patent application EP 0787465 A1 to Kim et al. teaches a cold plasma coagulator. The cold plasma coagulator of the reference includes a high frequency power supply, a gas dynamic block and a plasmotron. The power supply includes a rectifier, a capacitor storage and a voltage inverter. A resonance inductor and a dielectric tube are located coaxially with the one end of the dielectric tube being connected to the gas dynamic block and the second end of the dielectric tube in position to eject plasma through an output nozzle. The coils of the resonance inductor consist of a low voltage section and a high-voltage section, wherein the low voltage section of the resonance coils is connected to the output of the voltage inverter.
The German Democratic Republic patent 79087 to G. Pforr teaches a device for operating an inductive plasma torch. A high frequency alternating current derived from a high frequency generator is fed into an operating coil. The operating coil is disposed separately from the high frequency generator in an adjustable way and is connected by a power coaxial cable to the high frequency generator. An inductive plasma flame is generated within an insulating tube within the operating coil.
The European patent specification 0155496 to Peter Gagne teaches a plasma emissions source. The plasma emissions source comprises a radio frequency power generator, a plasma torch, means for automatically and continuously maximizing the radio frequency power transferred from the generator to a load coil. A series network and a shunt network are tunable for matching the impedance of the power generator and of the load coil. Each network includes at least one variable capacitor, a control means for driving the capacitors, wherein the control means includes a first motor means and a second motor means for driving said at least one capacitor of each network respectively, as well as a detector means for providing input signals representative of the phase relationship of the radio frequency voltage and the radio frequency current.
A hand piece for surgical cutting with a plasma jet beam is taught in U.S. Pat. No. 4,781,175 to Birtcher and shown in particular in FIGS. 6 to 9.
Conventional radio frequency surgery apparatus deliver unsteady signals exhibiting high current peaks of several amperes. These current peaks of conventional apparatus can cause damages also in deeper disposed tissue sections based on high electrical potential at the inside and at the outside of the cell membrane.
The recited problems of the known plasma generators have led in principle to a limitation in the application of electromagnetic technology in medicine and here particularly in the surgery field.